Synthetic composite fuel metering membrane

ABSTRACT

A synthetic composite flexible diaphragm of a fabric of warp-knitted polyester fibers and an elastomeric polymer coating applied only to one side of the fabric and disposed in interstices between the fibers to provide when cured a barrier at least substantially impervious to liquid fuel.

FIELD OF THE INVENTION

This invention relates to diaphragms and more particularly to animproved diaphragm and method of making diaphragms for carburetors andthe like.

BACKGROUND OF THE INVENTION

Diaphragm carburetors generally have a flexible diaphragm disposed in afuel chamber which opens to main and idling jets or orifices. Typically,the diaphragm divides the chamber into a wet chamber side which issupplied with fuel and is subjected to sub-atmospheric pressure duringoperation of the engine and a dry chamber side which may be subjected toatmospheric pressure. The diaphragm controls a fuel inlet valve disposedbetween a supply of fuel and the wet chamber side. As the engine drawsfuel from the wet chamber side, the quantity of the fuel in the wetchamber will decrease, and the diaphragm will move against the bias of aspring to open the needle valve and allow fuel to enter the chamber.

In operation, the diaphragm repeatedly opens and closes the inlet needlevalve so that fuel can enter the diaphragm chamber in response tosub-atmospheric pressure in the throat of the carburetor. Accordingly, acertain amount of fuel can be maintained in the diaphragm chamber at asubstantially constant pressure to supply fuel to the main and idlingjets. The diaphragm must be extremely flexible because the valve mustopen and close rapidly with a small pressure differential which istypically one to two inches of water. It must also operate over a widetemperature range of about -40° F. to 180° F.

For over 40 years the small engine industry has experienced manyproblems in the manufacture, in service use, and performance of fuelmetering diaphragms. Currently most diaphragms are made of a rubbercoated silk material. In use, the performance characteristics of thesediaphragms change and deteriorate substantially. The inventors havediscovered that the silk fibers absorb moisture from the atmosphereand/or the fuel, resulting in a diaphragm that changes its performancecharacteristics depending on the ambient weather conditions, such astemperature and humidity, and the moisture content of the fuel whichseverely limits the performance of the fuel metering diaphragms. Othermaterials, such as woven nylon, have also been found to be ineffectivebecause they produce a diaphragm which is too thick and/or inflexible toadequately respond to small pressure changes. When in prolonged contactwith liquid fuel, the coating also swells or increases in volume anddeteriorates in hardness, tensile strength and ultimate elongation.

Additionally, currently available manufacturing processes are unable toproduce diaphragms having the dimensional tolerances and performancecharacteristics required. A two-ply rubber coating is used with one plyor layer being applied to both sides of a sheet of material from whichthe diaphragm is cut. This increases the rigidity of the material which,when coupled with the above mentioned problems of the material, resultsin a diaphragm that in incapable of consistently performing as required.

Previously, the silk fabric with an uncured rubber coating on both sideswas heated in an oven to fully cure the rubber and then cooled to roomtemperature. Thereafter, a stack or pile of several of the resultingflat composite sheets were placed in a multiple cavity mold andsimultaneously molded under a force of 6-20 tons for a 54 cavity mold ata temperature of 330° to 375° F. for 4 to 8 minutes to form a bellows orconvolution in the diaphragm. Thereafter, the molded sheets were cooledand trimmed in a die and press to cut or blank out the individualdiaphragms from each sheet. When in service in a carburetor, theconvolution tends to become smaller, diminish or even disappear and thusis not permanent and degrades performance of the diaphragm.

Since up to four cured two ply rubber coated sheets are molded at thesame time to form the bellows, they are subjected to an unevenapplication of pressure and heat. This results in different performanceproperties for each diaphragm.

SUMMARY OF THE INVENTION

A fuel metering diaphragm embodying this invention has a warp knitfabric, preferably of polyester, with an elastomer coating applied toonly one side of the fabric and disposed in the interstices of thefabric to provide a fuel resistant barrier. The elastomer is applied toonly one side of a tensioned sheet of the fabric and one coated sheet ata time is molded under heat and pressure to dispose the elastomer in theinterstices of the fabric, vulcanize the coating on the fabric andpermanently form the desired contour of the diaphragm.

Objects, features and advantages of this invention are to provide adiaphragm having greater moisture resistance, enhanced flexibility,improved consistency when mass produced, greatly increased in serviceuseful life, flexibility and tensile strength, and of simple design andeconomical manufacture and assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of this invention willbe apparent from the following detailed description, appended claims andaccompanying drawings in which:

FIG. 1 is an end view of a carburetor assembly;

FIG. 2 is a side view of the carburetor assembly;

FIG. 3 is a view taken along line 3--3 in FIG. 1;

FIG. 4 is a top view of a diaphragm of the present invention with acutaway portion showing the tricot knit fabric;

FIG. 5 is a sectional view taken along line 5--5 of FIG. 4;

FIG. 6 is an end view of the tricot knit fabric stretched over a frame;

FIG. 7 is a fragmentary and enlarged top view of the fabric showing thewarp knit polyester fibers;

FIG. 8 is an end view of the stretched fabric with the elastomer coatingbeing applied with a doctor blade;

FIG. 9 is an end view of a heater assembly for the composite fabric andcoating;

FIG. 10 is an end view of a press and mold assembly;

FIG. 11 is an enlarged and fragmentary cross section of the fabric withthe elastomer coating applied to only one side and cured;

FIG. 12 is an end view of a trimming and cutting die and press; and

FIG. 13 is a semi-diagrammatic drawing of a preferred apparatus forapplying an elastomer coating to only one side of the fabric in largevolume production.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated in FIGS. 1-3, a conventional fuel pump and carburetor 20for a two cycle small engine has a main body 22 with a mixing passage 24in which a throttle valve 26 is mounted upon a shaft 28 controlled by alever 30. A fuel pump 32 in the body receives fuel from a fuel inlet 34and delivers it to a carburetor diaphragm chamber 36 through a diaphragmcontrolled inlet valve 38 in a conventional circuit including the fuelpump, the inlet valve, and a fuel metering diaphragm 40 embodying thisinvention. The carburetor is attached to the engine through mountingholes 42.

Small passages 44 opening into the mounting face of the carburetor (FIG.2) receive engine crankcase pressure pulses for actuating a diaphragm 46of the fuel pump received on the top of the carburetor body under a topplate or housing cap 47.

Referring to FIG. 3, in use, the pump 32 delivers fuel to the meteringvalve assembly 38 through a chamber 50 with a filter screen 52 thereinand a passage 54 terminating at a valve seat 56. The valve assembly hasa needle valve 58 which is actuated by a lever arm 60 connected at oneend 62 to the valve, fulcrumed between its ends at 64 and having acontrol finger 66 actuated at its free end by the diaphragm 40. Thediaphragm in cooperation with the body defines a fuel chamber 68 and incooperation with a cover plate 69 defines a dry or air chamber 70communicating with the atmosphere. The needle valve 58 is yieldablyurged to its closed position bearing on the seat 56 by a coil spring 72and is actuated to an open position by movement of the diaphragm 40. Thecoil spring is received in a pocket 73 in the body and bears on thefinger 66 of the lever arm.

Preferably, the diaphragm has a backing disc 74, and a plunger 76 with ashank extending through the disc, the diaphragm and a washer 78 whichare staked together in assembled relationship. Fuel in chamber 68 issupplied to a main metering jet 80 and idle jets 82 through passages(not shown) communicating with the fuel chamber 68. If desired, needlevalve assemblies 84 (only one of which is shown), can be provided toadjust the quantity of fuel supplied to the main jet and the idle jets.

In use, as fuel is drawn from the chamber 68, the quantity of fueltherein will decrease and the differential pressure on the diaphragmwill move the lever arm 60 against the bias of the spring 72 in acounter-clockwise direction (as viewed in FIG. 3), to open the valve 58to allow fuel to enter the chamber 68. As the chamber fills withadditional fuel, the diaphragm will tend to move the lever arm clockwiseand close the needle valve to thereby regulate the pressure of the fuelwithin the chamber. When the engine is under load, and particularly whenoperating at full throttle, fuel flows rapidly into the chamber whichcauses the diaphragm to cycle or fluctuate quite rapidly. Thus, for thecarburetor to function properly under a large variety of operatingconditions, it is necessary that the diaphragm be very flexible and yetstill be relatively strong and impervious to liquid fuel and notdeteriorate in use due to repeated flexing and contact with the liquidfuel.

The diaphragm 40 of the present invention, as seen in FIGS. 4 and 5,comprises a thin sheet of knit fabric 86 preferably of polyester whichhas been coated on only one side (preferably the fuel chamber 68 side)with an elastomer 88 and molded to form a typical bellows 90 with aconvolute shape. Desirably, a sealing gasket 89 is attached to thefabric adjacent its periphery and outside of the convolution.

One feature of the present invention is that the fabric 86 is preferablya warp knit fabric. A knit fabric is composed of a series ofinterconnecting loops which may be prepared in its simplest form from asingle yarn. By means of a needle, a loop is formed in a yarn 91 and iscast off when the needle draws a second loop through the first loop. Theprocess is then repeated, to form a chain of loops interconnecting withadjacent chains to form a fabric. FIG. 7 shows the basic structure ofthe warp knit fabric 86 with interconnecting loops of yarn 91 withinterstices 92 between the yarn. As viewed in FIG. 7, the lines of loopsrunning left to right are called "courses"; and those running from topto bottom are called "wales".

The knit fabric 86 is a plain warp knit fabric preferably of polyesterfibers. Warp knit and weft knit are the basic types of knit fabrics.Warp-knit fabrics are manufactured by passing each end of the fibersthrough its own needle forming a loop which intersects with adjacentloops. Most warp-knits are made on a tricot machine, as is known in theknitting industry. The resulting fabric is one of a tight weave havinggreat strength, flexibility and moisture resistance.

Preferably, the fabric is a tricot warp knit fabric of Dacron® polyesterfibers and has a nominal thickness of 0.004 to 0.010 and preferably0.005 to 0.007 of an inch as determined by American Society for TestingMaterials (ASTM) procedure D-1777, a weight of 0.5 to 3.0, desirably 0.6to 1.0, and preferably about 0.75 ounces per square yard as determinedby ASTM procedure D-3776 with the courses having 36 to 58, andpreferably 46 threads per inch, and the wales having 30 to 40 andpreferably 34 threads per inch, as determined by ASTM procedure D-3887.Preferably, this fabric has a transverse break strength of 10 to 20pounds and a longitudinal break strength of 15 to 25 pounds asdetermined by the grab test method of ASTM procedure D-5034. Preferably,the fabric has a Mullen burst strength of 45 to 75 psi as determined byASTM procedure D- 3786. Preferably, the threads of the fabric have amaximum denier of 20.

The tricot knit polyester fabric is coated on only one side with anelastomeric polymer, which is preferably a polyacrylonitrile butadienerubber (NBR). Preferably, this polymer is a medium acrylonitrilebutadiene copolymer that is compounded with various solvents,accelerators, nucleating agents, vulcanizing agents, plasticizers,release agents, etc., to optimize fuel resistance and flexibility.Preferably, the compounded NBR has a Money viscosity of 40 to 60,hardness on the Shore A scale of 30 to 40 points under ASTM procedureD-2240, and under ASTM procedure D-412, a tensile strength of at least1500 psi, an ultimate elongation of at least 800% and a Young's modulusof elasticity of at least 80 psi at 100% elongation.

Vulcanized samples of the compounded NBR after being soaked in thespecified fuel at room temperature for seventy hours preferably have thefollowing properties:

    ______________________________________                                                               ASTM D471                                                           ASTM Test Reference Fuel                                         Property       Procedure   B        C                                         ______________________________________                                        Maximum Hardness                                                                              D2240      10       10                                        Decrease in PTS                                                               Maximum Tensile                                                                              D412        60       80                                        Strength Decrease in %                                                        Maximum Ultimate                                                                             D412        35       50                                        Elongation Decrease in %                                                      Maximum Volume D471        20       35                                        Increase in %                                                                 Maximum Dryout D471        15       20                                        Volume decrease in %                                                          ______________________________________                                    

Other suitable materials are fabrics with warp knit synthetic fiberswith high moisture resistance such as fibers of polyaramid or polyamide(Nylon) having similar physical properties to the preferred polyestertricot knit fabric and elastomeric polymer coatings of fluorosiliconesor low temperature fluoroplastics or fluorocarbons (Viton® GFLT).

The knit fabric is coated on only one side with the elastomeric polymer.While being coated, preferably the fabric is tensioned or stretched to ataut and generally planar condition. To facilitate tensioning thefabric, preferably about one inch of the material on each side is heatedand fused to provide strips along the side edges for feeding andhandling the fabric. This keeps the material from "necking down" whentension is applied to the fabric during the coating process and producesa relatively stress-free finished composite membrane material formolding.

When a relatively small quantity of coated fabric is produced, a piece94 of fabric 86 may be tensioned or stretched to a taut and generallyplanar condition, as shown in FIG. 6, by stretching it over a carrierframe 96 and wrapping the ends of the piece about the frame. After apiece of fabric is tensioned and framed, a coat or thin layer 98 of thecompounded elastomer is applied to one side, preferably by using adoctor blade 100 as shown in FIG. 8 to produce a composite sheet ormembrane 102. The elastomer is applied relatively uniformly over onlyone surface or face 104 of the fabric. In applying the compoundedelastomer, only sufficient force is used to insure that the elastomeradheres to the fabric. Preferably, the wetting of the fabric by theelastomer is minimized so that the courses and wales of the fabric willnot be adhesively "locked up" which reduces the overall flexibility ofthe finished composite membrane. The fabric, not the elastomer, isbelieved to be the primary contributor to the flexibility of the moldeddiaphragm. After coating and before vulcanizing, the coated compositefabric preferably has a nominal thickness of about 0.012 to 0.018 of aninch.

To facilitate further handling, preferably before molding, the elastomeris allowed to develop a tacky condition by partially drying or slightlycuring it, which may be accomplished by exposure at room temperature orif desired by heating to an elevated temperature the composite sheet102. The composite sheet may be preheated by placing it in an oven 106,as shown in FIG. 9, for about 5 to 20 minutes with an oven airtemperature of about 150° F. to 200° F.

Thereafter, the coated sheet 102 is removed from the carrier frame,dusted with a conventional mold releasing agent, and cut into blanks 108of the appropriate size for the mold. For a mold 110 having 36 cavitieslaid out in a generally square (6×6) arrangement, these blanks aretypically about 10"×10".

As shown in FIG. 10, one blank 108 at a time is placed in the open mold110 which is closed by a press 112 to apply pressure and heat to thecomposite blank to force the elastomer into the interstices 92 betweenthe fibers of the knit fabric, to cure the elastomer and to produce thebellows or convolute shape in the finished diaphragm 40. The mold ispreheated to about 310° to 370° F. and preferably to 340° F. to 360° F.The composite blank has a residence time in the closed mold of about to41/2 minutes and preferably 31/4 to 33/4 minutes, while being subjectedto a molding force of 10 to 15 tons or about 220 to 330 pounds persquare inch of the area of one face of the composite blank received inthe mold. Preferably, to permit gases to escape from the mold, it isvented or it may be periodically rapidly slightly opened and then closedseveral times during the molding cycle.

This molding forces the elastomer on one side through the interstices 92of the fabric to the opposite side of the fabric and results in themolded composite sheet 108' of the fabric having an elastomeric coatingwhich is only one-ply deep but substantially covers the fibers on bothsides of the fabric. Typically, after the molding is completed, thesheet 108' of molded diaphragms has a nominal thickness of about 0.007to 0.010 of an inch.

The sheets 108' of molded diaphragms are cooled to room temperature andthen cut and trimmed in a die 114 (FIG. 12) received in a reciprocatingpress 116 which cuts the individual diaphragms 40 from a single moldedsheet 108' at a time. The individual finished diaphragms may then bepackaged for shipment, installation and use.

Desirably, but not necessarily, the sealing gaskets 89 are bonded to thecoated blank 108 during the molding operation. Individual gaskets may bedie cut from a sheet of high density cellulose fiber reinforced materialcoated on one side with a suitable bonding agent or adhesive. Thegaskets are placed in the mold on the blank 108 preferably with theadhesive facing the fabric side of the blank. The gaskets may be locatedin the mold with suitable locating pins projecting into the boltmounting holes through each gasket. A suitable cellulose gasket materialis commercially available as NV-512 from Specialty Paperboard, Inc. ofBeaver Falls, New York 13305. A suitable vulcanizable bonding agent iscommercially available as Thixon® OSN-2 from Whittaker, Dayton ChemicalsDivision, P.O. Box 127, West Alexandria, Ohio 45381.

FIG. 13 illustrates an apparatus 118 and method for coating with acompounded elastomer a relatively large quantity of a knit fabric. Athin sheet 120 of the compounded elastomer is produced by disposing aquantity of the compounded elastomer 122 between a pair of co-rotatingcylindrical rollers 124 and 126. The sheet 120 is stripped from theroller 126 by a stripper bar 128 and passed along with a web 130 of theknit fabric 86 between the nip of co-rotating cylindrical rollers 126 &132 to calender the sheet of elastomer onto only one face 134 of thefabric. Typically, the sheet of elastomer has a nominal thickness ofabout 0.008 to 0.010 of an inch and the gap between the calenderingrollers is adjusted to produce just enough force for the sheet ofelastomer to adhere to the fabric. The web 130 of fabric is rolled on areel 136 from which it is unwound as it passes between the calenderingrollers.

The web of coated fabric 130' is wound on a take-up reel 138 along witha polyethelene sheet 140 received on a reel 142 to prevent the layers ofuncured elastomer from sticking together. If desired, the web of coatedfabric 130' can be supported on one or more driven rollers 144 disposedbetween the take-up reel 138 and the calendering rollers. The relativespeed of the reel 136 unwinding the web 130 of fabric, the calenderingrollers 126 and 132, the support roller 144 and the take-up reel 138 canbe varied and adjusted within predetermined limits to properly tensionthe web 130 of fabric for application of the sheet 120 of elastomerthereto by the calendering rollers 126 and 132.

To facilitate substantially uniform tensioning of the web 130 of fabric,preferably each side edge is heated to provide a fused strip about 1"wide which is used in winding it on the reel 136, unwinding it, feedingit through the calender rollers, and winding it on the take-up reel 138.As previously indicated, these strips keep the fabric from necking downwhen tension is applied to the fabric during the coating process andminimize the stresses in the coated, molded and cured composite materialof the finished diaphragms 40.

Preferably, the polyethelene sheet 140 has a smooth surface whichpermits any residual stresses in the coated fabric web 130', due to ithaving been tensioned, to relax and dissipate before the coated fabricis molded. If desired, rolls of coated fabric 130' with the polyethylenesheet between the layer can be stored for several weeks before moldingby refrigerating it at a relatively low temperature of about 30° F. to40° F. to prevent curing of the compounded elastomer.

When desired, the web of coated fabric 130' can be unrolled and cut intoblanks of the appropriate size such as blanks 108, molded and trimmed inthe same manner as previously described for blanks 108 to producecompletely finished diaphragms 40.

The resulting diaphragms 40 are about 30% to 50% thicker and yetsubstantially more flexible than the prior art diaphragms made with asilk fabric. The resulting diaphragms have dramatically improvedmoisture and fuel resistance, durability and in service useful life,improved and more stable properties and dimensions, and are ofeconomical manufacture.

What is claimed:
 1. In a carburetor, a synthetic composite diaphragmhaving at least one side subject to contact with a liquid fuel, saiddiaphragm comprising: a fabric of warp knitted synthetic fibers withinterstices therebetween, and an elastomeric polymer coating applied toonly one side of the fabric, disposed in the interstices and cured toprovide a barrier at least substantially impervious to liquid fuel.
 2. Asynthetic composite diaphragm as defined in claim 1 whereinsaidelastomeric polymer coating is a polyacrylonitrile butadine rubber.
 3. Asynthetic composite diaphragm as defined in claim 1 whereinsaid one sideof the diaphragm with said elastomeric polymer coating applied thereonis subject to contact with liquid fuel.
 4. A synthetic compositediaphragm as defined in claim 1 whereinsaid elastomeric polymer extendsfrom said one side of the diaphragm through the interstices to theopposite side thereof.
 5. A synthetic composite diaphragm as defined inclaim 1 wherein said synthetic fibers of the knit fabric are polyesterfibers.
 6. A synthetic composite diaphragm as defined in claim 1 whereinsaid synthetic fibers of the knit fabric are selected from the groupconsisting essentially of at least one of polyester, polyamide,polyaramide fibers and blends thereof.
 7. A synthetic compositediaphragm as defined in claim 1 wherein said elastomeric polymer coatingconsists essentially of at least one of acrylonitrile butadiene,fluorosilicone and fluorocarbon polymers.
 8. A synthetic compositediaphragm as defined in claim 1 wherein said fabric has a nominalthickness not greater than about 0.009 of an inch as determined per ASTMprocedure D-1777.
 9. A synthetic composite diaphragm as defined in claim1 wherein said diaphragm has a nominal thickness not greater than about0.010 of an inch.
 10. A synthetic composite diaphragm as defined inclaim 1 wherein said fabric has a nominal thickness not greater than0.010 of an inch as determined per ASTM procedure D-1777.
 11. Asynthetic composite diaphragm as defined in claim 1 wherein saiddiaphragm has a nominal thickness not greater than about 0.010 of aninch.